An ink-jet printer includes a pen in which small droplets of ink are formed and ejected toward a printing medium. Such pens include printheads with orifice plates having very small nozzles through which the ink droplets are ejected. Adjacent to the nozzles inside the printhead are ink chambers, where ink is stored prior to ejection. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir part of the pen.
Ejection of an ink droplet through a nozzle may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. The rapid expansion of ink vapor forces a drop of ink through the nozzle. This process is called "firing." The ink in the chamber may be heated with a transducer, such as, a resistor that is aligned adjacent to the nozzle.
Thin-film resistors are conventionally used in printheads of thermal ink-jet printers. In such a thin-film device, the resistive heating material is typically deposited on a thermally and electrically insulated substrate. A conductive layer is then deposited over the resistive material. The individual heater elements (i.e., resistors) therein are dimensionally defined by conductive trace patterns that are lithographically formed using conventional masking, ultraviolet exposure and etching techniques on the conductive and resistive layers.
One or more passivation layers are applied over the conductive and resistive layers and then selectively removed to create a via for electrical connection of a second conductive layer to the conductive traces. The second "interconnect" conductive layer is patterned to define a discrete conductive path from each trace to an exposed bonding pad remote from the resistor. The bonding pad facilitates connection with a conductive lead from a flexible circuit that is carried on the pen. That circuit conveys control or "firing" signals from the printer's microprocessor to the resistors.
Materials providing passivation and cavitation barriers are layered over the resistive and conductive layers to complete the printhead substructure. The printhead substructure is overlaid with an ink barrier layer. The ink barrier is etched to define the shape of the ink chambers that are situated above, and aligned with, each resistor. An orifice plate overlays the ink barrier, with a nozzle opening to each chamber.
The resistors in the thin-film device are selectively driven by the above described thermo-electric integrated circuit part of the printhead substructure. The integrated circuit conducts the electrical signals from the printer microprocessor to the resistors, via the two conductive layers, to heat the resistors and create the super-heated ink bubbles for ejection from the chamber through the nozzle.
In summary, conventional thermal ink-jet printhead substructures require at least three major components be present in the firing chamber portion of the device: (1) a heater (resistor) layer, (2) a passivation (dielectric) layer, and (3) a cavitation barrier. Moreover, conventional ink-jet printhead substructures require at least four metal depositions to create the conductive and resistive layers, hence, requiring up to four source sputtering materials. Conventional printhead substructure fabrication also requires a double dielectrical deposition and at least five lithographic masks (excluding the ink barrier mask) in order to define the necessary thin-film IC components. Accordingly, conventional printhead substructure fabrication is both a labor intensive and an expensive process.
Current thermal ink-jet printhead substructures use aluminum as one of the basic components for the formation of the resistors and conductors. Although aluminum resistors and conductors are acceptable for most applications, they suffer from two major drawbacks: (1) electromigration, or physical movement, of the aluminum in the conductive traces which, in turn, causes reliability failures at relatively high current densities for both the resistor and the conductor, and (2) relatively complex fabrication processes. Also, conventional aluminum-based structures degrade rapidly at current densities greater than about 1.times.10.sup.6 amps/cm.sup.2.
A preferred embodiment of the present invention provides an ink-jet printhead substructure greatly simplified in both the method of manufacture and the resulting structure. The printhead substructure of the present invention comprises a resistor formed on an insulated substrate, a single conductor layer that provides both the interconnect paths and the conductive traces for the substructure, a passivation layer and a cavitation barrier.
The dual function (i.e., conductive interconnect paths and conductive traces) of the conductor layer of the present invention provides a greatly simplified printhead substructure. Additionally, the dual functioning conductor layer provides a simplified method of manufacture of the substructure as only one metal deposition is necessary.
In a preferred embodiment, the conductor layer is comprised of a noble metal, preferably palladium. A palladium conductor layer provides conductive traces with low resistance, a low rate of electromigration and excellent bonding properties.
Additionally, in a preferred embodiment of the present invention the resistor, passivation layer and cavitation barrier may comprise a single graded layer. This "graded thin-film structure" (GTFS) provides the resistor, passivation and cavitation barrier components without creating abrupt layer interfaces. Such abrupt, discrete component layers are typically the weaker areas in conventional printhead substructures and reduce printhead reliability and durability. Only a single sputter source material is needed to fabricate the GTFS.
Additionally, regardless of whether the resistor, passivation layer and cavitation barrier comprise discrete layers or a GTFS, fabrication of the printhead substructure of the present invention requires only two or three lithographic masks. With fewer masks, a thinner layer of conductive, passivation and cavitation barrier materials can be manufactured.
Printhead substructures comprised of thinner layers decrease thermal losses since thinner layers in contact with the substructure resistor reduces the typical thermal energy loss between the resistor and the ink. The passivation layer typically contributes the most to the substructure thermal inefficiencies due to its relatively low thermal conductivity characteristics. A more efficient thermal system, in turn, produces printheads with a lower turn-on-energy (TOE). Lower TOEs reduce printhead heating. Excessive printhead heating generates bubbles from air dissolved in the ink and causes prenucleation of the ink vapor bubble. Air bubbles within the ink and prenucleation of the vapor droplet result in a poor ink droplet formation and droplet volume control and thus, poor print quality.
The printhead substructure resistor of the present invention comprises a refractory metal, preferably tantalum-based. Refractory metal-based substructures do not suffer from the same electromigration problems as do aluminum-based systems. Moreover, refractory metal-based printhead substructures can operate at relatively high temperatures with minimal electrical or thermal degradation. Operation at higher temperatures allows an increase in print speed without sacrificing print quality.